PACAP and its receptor VPAC1 regulate megakaryocyte maturation: therapeutic implications

Megakaryocytes and platelets express the Gs-coupled VPAC1 receptor, for which the pituitary adenylyl cyclase-activating peptide (PACAP) and the vasointestinal peptide (VIP) are agonists. We here demonstrate a regulatory role for VPAC1 signaling during megakaryopoiesis. A total of 2 patients with trisomy 18p with PACAP overexpression and transgenic mice overexpressing PACAP in megakaryocytes have thrombopathy, a mild thrombocytopenia, and a reduced number of mature megakaryocytes in their bone marrow. In vitro differentiation of hematopoietic stem cells from the patient and transgenic mice shows a reduced number of megakaryocyte colonies compared with controls. The addition of PACAP, VIP, or the adenylyl cyclase activator forskolin to CD34(+) cells inhibits megakaryocyte differentiation. In contrast, neutralizing monoclonal anti-PACAP (PP1A4) or anti-VPAC1 (23A11) antibodies inhibit cAMP formation and stimulate megakaryopoiesis in a thrombopoietin-independent manner. Moreover, wild-type mice obtain an increased platelet count after subcutaneous injection of PP1A4 or 23A11. These antibodies also elevate platelet numbers in animal models of myelosuppressive therapy and in GATA1-deficient mice with congenital thrombocytopenia. Furthermore, 23A11 stimulates the in vitro megakaryocyte differentiation of both normal and GATA1-deficient human CD34(+) cells. Together, our data strongly suggest that VPAC1 signaling tempers normal megakaryopoiesis, and that inhibition of this pathway stimulates megakaryocyte differentiation, enhancing platelet recovery after myelosuppressive therapy and in GATA1 deficiency.